Pulsed solid state lasers are used extensively for material processing applications such as machining, drilling, and marking. Most commercially available, pulsed, solid-state lasers are operated by the well known technique of Q-switching. Q-switched pulsed lasers include a laser-resonator having a solid-state gain-element and selectively variable-loss device located therein. The laser-resonator is terminated at one end thereof by a mirror that is maximally reflecting at a fundamental wavelength of the gain-element, and terminated at an opposite end thereof by a mirror that is partially reflecting and partially transmitting at the fundamental wavelength. Such a laser is usually operated by continuously optically pumping the gain-element while periodically varying (switching) the loss caused by the variable loss device (Q-switch) between a value that will prevent lasing in the resonator and a value that will allow lasing in the resonator. While lasing is allowed in the resonator, laser radiation is delivered from the partially transmitting mirror as a laser pulse.
The pulse repetition frequency (PRF) of a Q-switched solid-state laser is determined by the frequency at which the Q-switch is switched. The pulse duration is determined for any particular gain-medium by factors including the transmission of the partially-transmitting mirror, any loss in the Q-switch in a lasing-allowed condition, the optical pump power, and the PRF. A pulse repetition rate and a pulse duration that are optimum for an operation on any one material will usually not be optimum for another operation or another material. Accordingly, an “ideal” pulsed laser would have independently variable PRF and pulse-duration to allow an optimum combination to be selected for most operations on most materials.
One type of pulsed solid-state laser in which the PRF can be varied without a variation in pulse duration is referred to by practitioners of the art as a cavity-dumped laser. In a cavity dumped-laser, a laser-resonator including a solid-state gain-element is terminated at each end thereof by a mirror that is maximally reflecting at a fundamental wavelength of the gain-element. Also included in the resonator is an optical switch, comprising a Pockels cell cooperative with a polarization-selective reflector (polarizing beamsplitter) and a quarter-wave plate. A Pockels cell includes a material, the birefringence of which can be switched by application of an electrical potential. The polarizing beamsplitter provides that only radiation plane-polarized in an orientation that is transmitted or reflected thereby can circulate in the resonator. The quarter-wave plate rotates the polarization plane of radiation by 90° degrees in a double-pass therethrough.
In one preferred mode of operation, the gain-element is continuously optically pumped. The polarizing beamsplitter provides a fold-mirror of the resonator. With no potential applied to the Pockels cell, the Pockels cell does not rotate the polarization plane of radiation so any radiation from the gain-element that is reflected by the polarizing beamsplitter will be transmitted out of the resonator by the polarizing beamsplitter after a double-pass through the Pockels cell and the quarter-wave plate. Accordingly, radiation can not circulate in the laser-resonator and the optical pumping builds up a population inversion in the gain-element.
When a laser-radiation pulse is required, a potential is applied to the Pockels cell sufficient to cause the birefringence of the Pockels cell to rotate the polarization-orientation of radiation by 90° in a double-pass therethrough. This can be described as a “quarter-wave state” of the cell. An additional 90° rotation of the polarization orientation is provided by the quarter-wave plate as noted above. Any radiation reflected by the polarizing beamsplitter will be re-reflected by the polarizing beamsplitter after a double-pass through the Pockels cell and the quarter-wave plate. Accordingly, radiation can circulate in the laser-resonator.
The circulation of the radiation in the resonator causes a build up of laser radiation in the laser-resonator. This depletes the population inversion in the gain-element which eventually limits the laser radiation building up in the resonator to some maximum level. At this point, the potential applied to the Pockels cell is switched off, and the laser radiation that has built up in the resonator (cavity) is reflected (dumped) out of the resonator within one round trip time for radiation in the resonator. This is only about a few nanoseconds (ns) for a resonator having a length between about 0.5 meters (m) and 1.0 m. It should be noted that the switching time for the Pockels cell from the no polarization-orientation rotation state to the quarter-wave state and vice-versa is about the round trip-time of the laser-resonator or less, for example a few nanoseconds.
It will be evident from the description provided above that the PRF of a cavity-dumped laser can be varied without varying the pulse-duration, as the pulse duration is determined by the resonator round-trip time. In certain applications, however, for example via-hole drilling, the few nanoseconds pulse duration of a cavity dumped laser is too short. It would be advantageous for these applications to have a cavity dumped laser in which the pulse duration could be made selectively longer, independent of the PRF.